1. Field of the Invention
The present invention relates to writing fluids for writing with a pen, and more particularly to aqueous-based ink for writing with ball point pens.
2. Description of Related Art
Aqueous ink compositions for ball-point pens must retain writability for a prolonged period of time with the cap removed from the pen as well as possess flow characteristics suitable for smooth writing with a line of uniform width corresponding to the ball diameter at the tip of the pen. In addition, the ink must flow to the tip of the pen without interruption when used for rapid writing without accumulating any residue of ink components at the tip of the pen.
Inks characterized as "paste" ink by virtue of high viscosity, normally in excess of 8,000 centipoise (cps), are utilized with a pen having a tube closed at one end by the ball, point assembly or a larger diameter tube, similarly closed at the point and with a viscous "follower" in intimate contact with the ink at the back of the tube to prevent back leaking and evaporation of the ink. The flow o characteristics, of paste ink in a ball point pen are dependent upon the viscosity of the ink and on the clearances between the ball and its housing.
Ball pens utilizing paste inks have at least two serious drawbacks. The high viscosity of the ink causes the pen to drag on the paper during writing, and the user must, therefore, press harder to generate a written line. Also,, the viscous ink tends to produce blots on the paper that smudge, or, to avoid smudging, the rate at which ink is metered onto the paper must be set so low that the intensity of the written line is too light.
To overcome these problems, a "fluid" ink can be used, one having a viscosity in the range of 1.0 to 10.0 cps. In a ball point pen utilizing fluid ink, the low viscosity of the ink provides free-flowing ink for writing at low pen-to-paper pressures and produces, in addition, a high intensity line. However, there are certain disadvantages inherent in the fluid-ink pen. First, the low viscosity ink does not provide adequate lubrication for the ball of the pen. Consequently, the pen is not smooth-writing, and the ball wears rapidly due to lack of lubrication. Secondly, the reservoir in the fluid ink ball pen must be vented directly to the atmosphere to prevent formation of a flow-inhibiting vacuum within the pen as ink flows from it or leakage when the pressure inside the pen increases due to changes in temperature. Fluid inks are generally water-based and are inherently subject to evaporation. In addition, fluid inks are prone to "feathering" due to absorption of the ink into the fibers of the paper by capillary action, giving a ragged look to the line.
Due to these problems, ball point pens utilizing fluid ink must employ a costly ink-flow control system that incorporates a capillary reservoir or a vacuum reservoir as taught in U.S. Pat. No. 3,951,555. In a capillary reservoir system, the ink reservoir includes a fibrous absorber, similar in design to a cigarette filter, which retains the ink within the reservoir by capillary attraction, supplying a flow of ink to the rotating ball through a feeder wick at a rate just sufficient to result in the formation of a continuous line during use, but insufficient to provide an excess supply of liquid ink at the writing tip. The feeder wick may employ porous rods in the form of sintered fibrous bundles or extruded plastic ink rods having capillary channels and intended to control the rate of ink feed to the rotating ball. The reservoir, a bundle of fibers, is connected to a feeder rod with the fiber bundle acting as a reservoir to hold ink inside the pen, and with the other end of the rod in close proximity to the tip of the pen. Inks containing pigments rater than soluble dyes tend to clog the capillary passages in fibrous reservoirs and feed rods. Examples of prior art patents exemplifying such systems include U.S. Pat. Nos. 4,145,148 and 4,726,845.
Another disadvantage of the design utilizing a fiber reservoir and feeder rod is that up to 50% of the ink may be retained by the fibrous reservoir and never be available for writing because it is retained in the smaller capillary channels between the fibers. U.S. Pat. No. 4,671,691 reports that the problems associated with the fibrous reservoir and feeder rod assembly cannot be corrected by simply thickening the ink and eliminating the assembly. Ink flow at the tip of the pen can be lost due to evaporation or mechanical shocks, that create a separation of ink from the point of the pen.
An alternative Solution to the problems associated with elimination of the fibrous reservoir and feeder rod is provided by ink containing a polar solvent that is at least 50% by weight of water and is thickened by a water-dispersible gum or resin. For instance, Great Britain Patent No. 2,131,040A teaches that an ink with at least 32.74% water, 0.2 to 0.45% xanthan gum as thickener, and at least 5% of a wetting agent will overcome the problems inherent with ball pens employing the fibrous reservoir and feeder rod assembly. U.S. Pat. No. 5,045,992 describes a shock resistant ink that is at least 50% water and contains a polymeric thickener with a shock resisting additive, such as polyvinyl pyrrolidone, and/or polyvinyl alcohol. U.S. Pat. No. 5,013,361 teaches that at least 49.5% water is needed when a polymeric thickener is used in ink for a ball pen.
U.S. Pat. No. 4,942,185, describes an ink for marker pens that is at least 50% water and is stabilized by a water solubilized terpolymer of .alpha.-methylstyrene, styrene, and acrylic acid. The terpolymer is disclosed as providing the attribute of shear thinning, thereby permitting flow through the fiber tip of the marker pen.
The concept that the thickness of ink must change as the ink moves through the pen onto the surface of the ball is well known in the prior art. It is known to incorporate into the ink a polymeric material that converts the ink to a shear-thinning fluid, allowing it to act as a liquid under high shear conditions and as a paste under conditions of low shear. U.S. Pat. No. 3,425,779 teaches that the rheological properties of the ink must be designed to control flow through the tip of the ball pen and it utilizes a soluble polymer to provide a desired thickness to the ink.
For example, U.S. Pat. No. 4,726,845 provides a writing ink that displays a high shear viscosity allowing greater smoothness in writing and less wicking than typical liquid ink ball pen inks. British Patent Application No. 2,131,040 and U.S. Pat. No. 4,671,691 disclose an aqueous ink containing 0.2 to 0.45 part by weight of xanthan gum that is used in a ballpoint pen that includes a viscous follower in the ink reservoir. British Patent Application No. 2,094,820A discloses an ink composition comprising a polyelectrolyte polymer and a high polarity solvent that is used like a liquid ink in a ball pen having a sealed and pressurized ink reservoir. More recently, a "substantially waterless" writing ink thickened with xanthan gum is disclosed in U.S. Pat. No. 4,726,845. The ink has high viscosity while at rest but thins out quickly if subjected to a shearing force such as is found in the socket of a roller ball pen as the ball rotates.
Every pen design has different geometries for the ball and socket that significantly change the flow characteristics of the ink. The optimum viscosity and flowability of the ink is unique to each pen design. Thus, the best "rheometer" for testing any ink is a use test conducted with the pen that will be used by the customer.
A particularly troublesome problem associated with inks containing substantial amounts of water and other volatile liquids is known as the "cap off" problem in which a pen fails to write if it is stored without a cap around the tip. Without the cap to prevent evaporation, the ink dries, and debris of resin, pigment and other components do not flow upon application of shear stress, forming a crust around the ball and at the interface of the ball socket. This debris blocks the ink from flowing into the tip of the pen. The problem is particularly acute when the ink contains water, which evaporates quickly.
Consequently, the need exists for more and better formulations of ink for use in ball pens, particular of shear-thinning inks. Particularly, needed are formulations of shear-thinning inks containing substantially reduced water content to minimize the "cap off" problems generally associated with water-containing inks.